1. Field of the Invention
The present invention relates to a silver halide emulsion, a method of preparing the emulsion, and a silver halide color photographic photosensitive material and an image forming method using the emulsion. More specifically, the present invention relates to a silver halide emulsion, which exhibits high sensitivity, high contrast, little sensitivity variation with humidity conditions at the time of exposure, and excellent reciprocity law properties at high illumination intensities, to a method of preparing the emulsion in a stable manner, and to a silver halide color photographic photosensitive material and an image-forming method using the emulsion. Further, the present invention relates to a silver halide emulsion which provides high-contrast gradation at high sensitivity even by digital exposure such as exposure by laser scanning, and to a silver halide color photographic photosensitive material using the emulsion.
2. Description of the Related Art
In recent years, in the field of color photographic papers there has been an increasing demand for further improvements of such characteristics as sensitivity, image quality, and toughness at the time of processing. Accordingly, there is demand for an emulsion that provides high contrast and high sensitivity and exhibits little variation in photographic properties even under different temperature and humidity conditions at the time of exposure. Meanwhile, because of recent widespread use of laser scanning exposure apparatuses, suitability to short-time exposure at high illumination intensities has become an important characteristic. A remarkable feature of laser scanning exposure is that it enables speedup of exposure and improvement of resolution. However, when laser scanning exposure is applied to color photographic papers, suitability to exposure in very short times which have hitherto not been experienced (specifically, 10−6 seconds) is required at high illumination intensities.
Gold-sensitization is effective as a means for raising the sensitivity of a silver halide emulsion. As a gold compound for use in the gold-sensitization (occasionally, hereinafter referred to as “gold sensitizer”), a gold (I) compound containing a mesoionic ligand (this compound is hereinafter referred to as a “mesoionic gold (I) compound”) is known. Japanese Patent Application Laid-Open (JP-A) No. 4-267249 discloses that a mesoionic gold (I) compound is useful for the manufacture of a high-sensitivity and high-contrast emulsion. However, as described in JP-A No. 11-218870, the mesoionic gold (I) compound is known to have low stability in a solution. Since stability of the gold sensitizer in a solution is an essential property for the production of an emulsion of reliable qualities in a stable manner, there is a demand for improvement of the stability.
As a measure to solve this problem, JP-A No. 11-218870 proposes a method in which a gold (I) complex of a mercapto compound is utilized as a gold sensitizer. However, although the solution stability of this gold sensitizer is improved, this gold sensitizer is still a compound that decomposes in a solution and therefore it does not satisfactorily solve the problem.
Besides the utilization of the gold (I) compound described above, it is also known to utilize an Au (III) compound such as chloroauric acid or the like. Since chloroauric acid is sufficiently stable in an aqueous solution, the use of chloroauric acid makes it possible to alleviate the productivity-related problem due to instability of the gold (I) complex. However, since the photographic properties such as sensitivity, gradation, suitability to high illumination intensity exposure, and toughness with respect to environmental temperature and humidity at the time of exposure are unsatisfactory, there is a demand for improvement. Accordingly, a means for producing, at a constant quality in a stable manner, a silver halide emulsion which satisfies the above-mentioned photographic properties has not been proposed and therefore there is a demand for solution of this problem.
In view of raising the toughness under various processing conditions, in particular toughness with respect to wet abrasion, JP-A No. 10-123658 discloses that an emulsion containing a specific disulfide compound is effective. However, since the photographic properties such as sensitivity, gradation, suitability to high illumination intensity exposure, and toughness with respect to environmental temperature and humidity at the time of exposure are particularly unsatisfactory in connection with the gold sensitizer comprising an Au (III) compound, there is demand for improvement.
Meanwhile, because of the requirement for rapid processability, mainly for higher productivity, silver halide emulsion having a high silver chloride content is used in color photographic papers. Generally, such silver halide emulsion having a high silver chloride content tends to exhibit low sensitivity and soft tones after high illumination intensity exposure. Accordingly, a variety of techniques to alleviate this problem have been disclosed.
In order to alleviate high illumination intensity reciprocity failure of a silver halide emulsion, it has been known to dope iridium. However, silver chloride emulsion doped with iridium is known to cause latent image sensitization shortly after exposure. For example, according to Japanese Patent Application Publication (JP-B) No. 7-34103, the problem of latent image sensitization is solved by providing a localized phase having a high silver bromide content and doping this phase with iridium. The silver halide emulsion prepared by this method exhibits high sensitivity and high contrast even with a relatively high illumination intensity exposure of about 1/100 seconds. However, this silver halide emulsion has been found to pose a problem that it is difficult to obtain high-contrast gradation while maintaining high sensitivity up to ultrahigh illumination intensity exposures of 1μ second, which are required in a digital exposure system with laser scanning exposure. U.S. Pat. No. 5,691,119 discloses a method for preparation of an emulsion made up of grains having localized phases rich in silver bromide to obtain high-contrast gradation in high illumination intensity exposure. However, the effect of this method is not sufficient and a problem of this method is that performance is not stable when the preparation is repeated.
U.S. Pat. Nos. 5,783,373 and 5,783,378 disclose a method to alleviate the high intensity reciprocity failure and produce high contrast, by use of at least 3 kinds of dopants. But this high-contrast gradation is obtained by use of dopants that produce high contrast by desensitization and therefore this method is incompatible with the creation of high sensitivity in principle.
U.S. Pat. Nos. 5,726,005 and 5,736,310 disclose an emulsion having high sensitivity and little high intensity reciprocity failure, obtained by including a silver chloride-rich emulsion containing iodine having a peak concentration on the grain subsurface. This emulsion exhibits higher sensitivity as the intensity of illumination for exposure becomes higher, but it was found that the gradation was very soft and the emulsion was not suitable to digital exposure in which the dynamic range of light amount is limited.
U.S. Pat. No. 5,049,485 discloses that chemical sensitization by an Au (I) compound having a mesoionic coordination leads to high sensitivity and high contrast. U.S. Pat. No. 5,945,270 discloses that chemical sensitization by an Au (I) compound having a water-soluble group-bearing mercapto coordination leads to high sensitivity and high contrast. These Au (I) compounds are known to be relatively stable, but no mention is made as to whether use of these Au (I) compounds is suitable for high illumination intensity exposure.